In recent years, in a mobile communication system, a variety of methods for allocating transmission channels have been proposed for efficient use of frequencies, of which some have been put into practical use.
FIG. 6 shows channel arrangements in a variety of communication systems: Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Path Division Multiple Access (PDMA).
First, referring to FIG. 6, brief description of FDMA, TDMA and PDMA will be provided. FIG. 6(a) shows FDMA, in which analog signals of users 1 to 4 are transmitted in a frequency-divided manner with radio waves of different frequencies f1 to f4, and the signals of respective users 1 to 4 are separated by frequency filters.
In TDMA shown in FIG. 6(b), digitized signals of respective users are transmitted in a time-divided manner at a certain time period (time slot) with radio waves of different frequencies f1 to f4. The signals of respective users are separated by frequency filters and time synchronization between a base station and respective user mobile terminals.
Meanwhile, a PDMA system has recently been proposed in order to improve the efficiency of the use of radio wave frequencies as mobile phones have widely been used. As shown in FIG. 6(c), in the PDMA system, one time slot in the same frequency is spatially divided, and data of a plurality of users are transmitted. In the PDMA system, signals of respective users are separated, using frequency filters, time synchronization between the base station and respective user mobile terminals, and a mutual interference eliminator such as an adaptive array.
An operation principle of such an adaptive array radio base station is well-known, as described in the references below.
B. Widrow, et al. : “Adaptive Antenna Systems,” Proc. IEEE, vol. 55, No. 12, pp.2143–2159 (December 1967).
S. P. Applebaum: “Adaptive Arrays”, IEEE Trans. Antennas & Propag., vol. AP-24, No. 5, pp.585–598 (September 1976).
FIG. 7 is a schematic diagram conceptually showing the operation principle of such an adaptive array radio base station. In FIG. 7, one adaptive array radio base station 1 includes an array antenna 2 consisting of n antennas #1, #2, #3, . . . , #n, of which coverage is shown with a first hatched region 3. On the other hand, coverage of adjacent, another radio base station 6 is shown with a second hatched region 7.
In region 3, a radio wave signal is communicated between a mobile phone 4, which is a terminal for a user A, and adaptive array radio base station 1 (arrow 5). Meanwhile, in region 7, another radio wave signal is communicated between a mobile phone 8, which is a terminal for another user B, and radio base station 6 (arrow 9).
Here, if the frequency of the radio wave signal of mobile phone 4 for user A is by chance equal to that of mobile phone 8 for user B, the radio wave signal from mobile phone 8 for user B may be an unnecessary, interfering signal within region 3, depending on a position of user B, and it may cross with the radio wave signal between mobile phone 4 for user A and adaptive array radio base station 1.
As described above, adaptive array radio base station 1 that has received the crossed radio wave signals from both users A and B will output a crossed signal, unless the signal is subjected to some kind of processing. In such a case, communication of user A, that should originally be established, will be prevented.
[Configuration and Operation of Conventional Adaptive Array Antenna]
In order to eliminate the signal from user B from an output signal, adaptive array radio base station 1 carries out a processing in the following. FIG. 8 is a schematic block diagram showing a configuration of adaptive array radio base station 1.
Initially, the signal from user A is represented as A(t), and the signal from user B is represented as B(t). Then, a reception signal x1(t) at the first antenna #1 constituting array antenna 2 in FIG. 7 is expressed as follows.x1(t)=a1×A(t)+b1×B(t)
Here, a1 and b1 are coefficients which vary in real time, as described later.
Similarly, a reception signal xn(t) at the nth antenna #n is expressed as follows.xn(t)=an×A(t)+bn×B(t)
Here, an and bn are also coefficients that vary in real time.
The aforementioned coefficients, a1, a2, a3, . . . , an indicate that there will be differences in reception intensity at respective antennas. This is because relative positions of antennas #1, #2, #3, . . . , #n constituting array antenna 2 are different with respect to the radio wave signal from user A respectively (for example, each antenna is arranged with a space from one another by a distance of 5 times of a wavelength of the radio wave signal, that is, approximately 1 meter).
In addition, similarly, coefficients b1, b2, b3, . . . , bn indicate that there will be differences in reception intensity at respective antennas #1, #2, #3, . . . , #n, with respect to the radio wave signal from user B. Since each user travels, these coefficients vary in real time.
Signals x1(t), x2(t), x3(t), . . . , xn(t) received by respective antennas enter a reception portion 1R constituting adaptive array radio base station 1 via corresponding switches 10-1, 10-2, 10-3, . . . , 10-n, and are provided to a weight vector control portion 11 as well as to one inputs of corresponding multipliers 12-1, 12-2, 12-3, . . . , 12-n.
Weights w1, w2, w3, . . . , wn with respect to the reception signals at respective antennas are applied to the other inputs of these multipliers from weight vector control portion 11. These weights are calculated in real time by weight vector control portion 11, as described below.
Therefore, reception signal x1(t) at antenna #1 will be w1×(a1A(t)+b1B(t)) through multiplier 12-1, reception signal x2(t) at antenna #2 will be w2×(a2A(t)+b2B(t)) through multiplier 12-2, reception signal x3(t) at antenna #3 will be w3×(a3A(t)+b3B(t)) through multiplier 12-3, and further, reception signal xn(t) at antenna #n will be wn×(anA(t)+bnB(t)) through multiplier 12-n.
Outputs of multipliers 12-1, 12-2, 12-3, . . . , 12-n are added in an adder 13, of which output is represented as follows.w1(a1A(t)+b1B(t))+w2(a2A(t)+b2B(t))+w3(a3A(t)+b3B(t))+ . . . +wn(anA(t)+bnB(t))
When this representation is divided into two terms, that is, a term relating to signal A(t) and a term relating to signal B(t), the following representation can be obtained.(w1a1+w2a2+w3a3+ . . . +wnan)A(t)+(w1b1+w2b2+w3b3+ . . . +wnbn)B(t)
Here, adaptive array radio base station 1 distinguishes between users A and B, and calculates the above weights w1, w2, w3, . . . , wn so as to extract only the signal from a desired user, as described below. For example, in an example of FIG. 8, weight vector control portion 11 regards coefficients a1, a2, a3, . . . , an, b1, b2, b3, . . . , bn as constants, in order to extract only signal A(t) from user A with which communication is to be established. Further, weight vector control portion 11 calculates weights w1, w2, w3, . . . , wn such that coefficients for signal A(t) attain 1 as a whole, and coefficients for signal B(t) attain 0 as a whole.
In other words, weight vector control portion 11 calculates, in real time, such weights w1, w2, w3, . . . , wn that coefficient of signal A(t) attains 1 and coefficient of signal B(t) attains 0, by solving simultaneous simple equations below.w1a1+w2a2+w3a3+ . . . +wnan=1w1b1+w2b2+w3b3+ . . . +wnbn=0
Though description for how to solve these simultaneous simple equations will not be provided, it is well known as described in the aforementioned references, and has already been put into practical use in the adaptive array radio base station.
By setting weights w1, w2, w3, . . . , wn as described above, an output signal of adder 13 will be given as shown below.Output signal=1×A(t)+0×B(t)=A(t)[User Identification and Training Signal]
It is to be noted that users A and B above are distinguished in the following manner.
FIG. 9 is a schematic diagram showing a frame configuration of a radio wave signal of a portable phone. The radio wave signal of the mobile phone is mainly composed of a preamble consisting of a signal sequence already known to the radio base station, and data (such as voice data) consisting of a signal sequence unknown to the same.
The signal sequence of the preamble includes those of information determining whether or not the user is a desired user with which the radio base station should communicate. Weight vector control portion 11 (FIG. 8) in adaptive array radio base station 1 compares a training signal corresponding to user A, taken out from a memory 14, with the received signal sequence, and controls the weight vector (determines weights) so as to extract a signal that appears to include the signal sequence corresponding to user A. The signal of user A thus extracted is output to the outside from adaptive array radio base station 1 as an output signal SRX(t).
On the other hand, in FIG. 8, an external input signal STX(t) enters a transmission portion 1T constituting adaptive array radio base station 1, and is provided to one inputs of multipliers 15-1, 15-2, 15-3, . . . , 15-n. Weights w1, w2, w3, . . . , wn previously calculated based on the reception signal by weight vector control portion 11 are copied and applied to the other inputs of these multipliers respectively.
The input signals weighted by these multipliers are sent to corresponding antennas #1, #2, #3, . . . , #n via corresponding switches 10-1, 10-2, 10-3, . . . , 10-n, and transmitted to region 3 in FIG. 7.
Here, the signal to be transmitted with the same array antenna 2 as used in reception is weighted, also targeted for user A as the reception signal. Therefore, the transmitted radio wave signal is received by mobile phone 4 for user A as if it has directivity toward user A.
FIG. 10 visualizes such communication of the radio wave signal between user A and adaptive array radio base station 1. In contrast to region 3 in FIG. 7 showing an area which the radio wave can actually reach, a state in which the radio wave signal is emitted with directivity, targeted for mobile phone 4 for user A, from adaptive array radio base station 1 is imaged, as shown in a virtual region 3a of FIG. 10.
In the PHS, which is a digital mobile communication system, the adaptive array as described above has already been put into practical use, and implementation of the PDMA system which can accommodate a larger number of users has been discussed. Such a PDMA system is disclosed in the following references.
(1) Suzuki, Hirade, IEICE Technical Report, vol. RCS93-84, pp.37–44, January 1994
(2) S. C. Swales, M. A. Beach, D. J. Edwards, J. P. McGeehan, IEEE Trans. Veh. Technol., vol. 39, pp.56–67, Febuary 1990
(3) T. Ohgane, Y. Ogawa, and K. Itoh, Proc. VTC '97, vol. 2, pp.725–729, May 1997
As described above, in the PDMA system (Path Division Multiple Access) using the adaptive array, an identical channel can be allocated to a plurality of users in the same cell by adaptively directing null of array antenna directivity to an interfering user, so long as an optimal weight vector is calculated.
Thus, in the PDMA system, a technique to eliminate interference in the identical channel is required. In this regard, the adaptive array adaptively directing null to an interfering wave is effective, because the interfering wave can effectively be suppressed even if the level of the interfering wave is higher than that of a desired wave.
When the adaptive array is used in the base station, unnecessary emission in transmission can also be reduced, in addition to eliminating interference in reception.
Here, for an array pattern in transmission, an array pattern in reception may be used, or alternatively, a new array pattern may be generated from a result of estimate of an incoming direction and the like. The latter method is applicable, regardless of FDD (Frequency Division Duplex) or TDD (Time Division Duplex), however, a complicated processing will be necessary. On the other hand, when the former method is used for FDD, correction for array arrangement, weight or the like will be needed, because array patterns are different in transmission and reception. Therefore, application for TDD is generally assumed, and satisfactory property has been obtained in an environment where external slots are continuous.
As described above, in the TDD/PDMA system using the adaptive array in the base station, when an array pattern (a weight vector pattern) obtained in an uplink is used for a downlink, and if a dynamic Raleigh propagation path with spread angle is assumed, an error rate may be deteriorated in the downlink due to a time difference between the uplink and the downlink.
In other words, there is an interval from a time point when the radio wave is transmitted from a user terminal to the base station through the uplink to a time point when the radio wave is emitted from the base station to the user terminal through the downlink. Therefore, an error between the emitted direction of the radio wave from the base station and a direction in which the user terminal is actually present will deteriorate the error rate, when a traveling speed of the user terminal is not negligible.
As a method for estimating a weight for the downlink taking into account fluctuation of such a propagation path, a technique of first order extrapolation using a weight vector value obtained in the uplink has been proposed in the following references.
(1) Katoh, Ohgane, Ogawa, Itoh, IEICE Trans., vol. J81-B-II, no. 1, pp. 1–9, January 1998.
(2) Doi, Ohgane, Karasawa, IEICE Technical Report, RCS97-68, pp.27–32, July 1997.
When change over time in the weight is actually observed, however, it is not linear, and the error tends to be large with the conventional technique of the first order extrapolation of the weight vector.
In addition, in estimating the weight in transmission, it is also necessary to enable processing with a practical circuit scale.